The use of robot arms for positioning and placing objects is well known. Generally, the arms have Z-, R- and .theta.- motion in a conventional cylindrical coordinate system. The capability of providing straight line motion is very important in the processing of semiconductor wafers so as to allow them to be very accurately positioned at a work station where processing steps take place. The R or straight line movement of the end effector or mechanical hand at the end of the arm has been accomplished in a number of manners.
As one example, telescoping arms have been utilized for this purpose. In such a structure one slidable member fits within another thus allowing linear extension of the arm.
More commonly, two link arms with equal length links have been utilized for this purpose. The links are connected to each other so that distal end of the first link is pivotally attached to the proximal end of the second link. The links utilize belt drives which are provided for coordinately rotating the second link to the first link to provide a rotation ratio, i.sub.2,1 of 2/1, and to provide a rotation ratio, i.sub.3,2, of 1/2 between the end effector and the distal link of the robotic arm. When i.sub.2,1 is equal to 2/1 and i.sub.3,2 is equal to 1/2, the result is that i.sub.31, the rotation ratio of the end effector relative to the first link, is equal to 2/1.times.1/2 or unity and straight line motion results. In the case of 3 link arms, such as those shown in U.S. Pat. No. 5,064,340, the rotation ratio between the third and second links is 1/1 and other ratios are as just discussed above. In this situation i.sub.21 is equal to 2/1, i.sub.3,2 is equal to 1/1 and i.sub.4,3 is equal to 1/2. This assures that i.sub.4,1 is equal to unity and straight line motion results.
United Kingdom Patent Application GB 2193482A, published Feb. 10, 1988 discloses a wafer handling arm which includes two unequal length links with the distal end of one link being pivotally attached to the proximal end of the other link, with the hand being integral with the distal end of the distal link and which utilizes a belt drive which is fixed to a cam to attain nearly straight line motion.
It is also known to utilize an isosceles triangle type linkage wherein two equal length links are pivoted together and a mechanical hand is pivoted to the distal end of the distal link. Pulleys and belts are utilized in such a manner that the angle between the two links changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted. This linkage provides drive directly from a motor shaft to the proximal end portion of the proximal link. A belt about a stationary pulley coaxial with the motor shaft passes about a pulley at the point of pivoting of the two links to one another. Another pulley and belt arrangement provides pivoting of another pulley where the second link is pivotally connected to the mechanical hand.
In another apparatus a pair of isosceles triangle type linkages face one another and the mechanical hand is pivotally attached to the distal ends of both of the distal links. The proximal ends of each of the proximal links is driven in an opposite direction of rotation by a single rotating motor shaft, generally through use of appropriate gearing. What results is a frogs leg type of motion with each isosceles triangle type linkage serving as means for controlling the other such linkage in such a manner that the angles between the two links of each of the isosceles triangle linkages changes at twice the rate as do the angles that each of the links makes with a line connecting the points about which their other ends are pivoted.
There are a number of situations where it would be desirable to be able to tilt a robotic arm mechanism, as a whole, so as to allow better alignment for a particular task. One particular situation which comes to mind is the unloading of semiconductor wafers from cassettes. A typical semiconductor wafer cassette has a plurality of semiconductor wafers stacked one above the other and spaced apart from one another. Each wafer is generally held at its periphery by a narrow ledge with the great majority of the area of the wafer being untouched so as to avoid contamination or other problems. The wafers are generally loaded in the cassettes mechanically and occasionally one or more wafers in a cassette may end up misaligned, that is, canted, with one portion of the periphery upon a higher ledge than the diametrically opposite portion. In such an instance current robotic arms utilized in the semiconductor industry are not able to compensate and pick up the canted semiconductor wafer and deliver it for processing. This results in lost production. Furthermore, even if the wafers are originally loaded properly on the cassette, as the cassette is utilized, i.e., as it is, for example, advanced upwardly so that the next semiconductor wafer is ready for being picked up by a robotic arm, this very motion can cause one or more of the semiconductor wafers to be jiggled off of one of the ledges whereby the result is a semiconductor wafer having one edge held on a ledge and having the opposite edge at a lower level and bearing against the top of another semiconductor wafer. Prior to the present invention an adequate solution to this problem has not been available.